1. Field of the Invention
The present invention relates generally to a power distribution unit with individual GFI modules and a line supervisory circuit, and more particularly pertains to a power distribution unit with individual GFI modules and a line supervisory circuit wherein individual outlets (120-125 VAC or 240-250 VAC) are protected by individual Ground Fault Interrupter (GFI) modules, and a line supervisory module is provided for line fault protection.
2. Discussion of the Prior Art
Conventional Ground Fault Circuit Interrupter (GFCI) devices are designed to trip in response to the detection of a ground fault condition at an AC load. Generally, the ground fault condition results when a person comes into contact with the line side of the AC load and an earth ground at the same time, a situation which can result in serious injury. The GFCI device detects this condition by using a sensing transformer to detect an imbalance between the currents flowing in the line and neutral conductors of the AC supply, as will occur when some of the current on the line side of the AC load is being diverted to ground. When such an imbalance is detected, a circuit breaker within the GFCI device is immediately tripped to an open condition, thereby opening both sides of the AC line and removing all voltages from the load. Many types of GFCI devices are capable of being tripped not only by contact between the line side of the AC load and ground, but also by a connection between the neutral side of the AC load and ground. The latter type of connection, which may result from a defective load or from improper wiring, is potentially dangerous because it can prevent a conventional GFCI device from tripping at the intended threshold level of differential current when a line-to-ground fault occurs.
Although GFCI receptacles of the type described above can provide useful protection against electrical shock hazards, they are designed to trip only in response to a ground fault condition and not in response to other types of conditions which can also be dangerous. For example, although a broken neutral conductor in an AC power source will de-energize the load, the line side of the AC source is still energized and can create a shock hazard at the load. A conventional GFCI device will not detect this condition, since the broken neutral conductor disables the control circuitry within the GFCI device and renders it incapable of tripping.
Portable GFCI devices have been designed for use in situations where the available AC power supply circuit does not include a central or receptacle-type GFCI device. These portable devices may be incorporated into line cords, extension cords or plug-in units, and are often used with power tools and other types of potentially hazardous power equipment at construction sites and the like.
Temporary branch power, as is frequently employed at temporary construction sites, provides and utilizes temporary electrical power and lighting wiring methods which may be of a class less than would be required for a permanent installation.
However, the National Electrical Code, as implemented by OSHA (Occupational Safety and Health Administration) requires that all 15 and 20 amp receptacle outlets on single phase circuits for construction sites have approved ground fault circuit protection for personnel. Accordingly, OSHA requires all temporary branch power systems to be GFI protected.
Two common problems can occur at construction sites with power supply cables that feed portable power boxes. Abuse or mis-wiring can cause 1) reversed power lines (L.sub.1 or L.sub.2) and neutral (N) or 2) an open neutral (N). These two problems can be dangerous when using a single pole GFI/breaker in portable power boxes, as either one of these problems can deprive workers of needed GFI protection.
For instance, when the neutral (N) and line 1 (L.sub.1) are reversed and should a fault occur, the neutral is opened, not line 1 (L.sub.1), and the fault current still flows. The worker is unprotected. It is also possible under these circumstances to have 240 V supplied to a 125 V receptacle which creates an additional problem by possibly damaging the GFCI which would eliminate the protection thereof and present a potential danger.
Under circumstances when the neutral (N) is open, power is being fed to the receptacles, but power is not being fed to the neutral (N) terminal and the single pole GFI circuit breaker will not trip. The worker can be subjected to line voltage and thus be unprotected. From these two examples, it is easy to see the potential danger in using single pole GFI/circuit breakers in portable equipment.